A galaxy is a huge system of billions of stars, and clouds of dust and gases.Millions of such galaxies make up the universe.
The Universe is all of space and time (space time) and its contents,which includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space and all matter and energy.The size of the entire Universe is still unknown.
celestial bodies :-
The sun, the moon and all those objects shining in the night sky are called celestial bodies.
Some celestial bodies are very big and hot. They are made up of gases. They have their own heat and light, which they emit in large amounts. These celestial bodies are called stars. The sun is a star.
While watching the night sky, you may notice various patterns formed by different groups of stars. These are called constellations. Ursa Major or Big Bear is one such constellation. One of the most easily recognisable constellation is the small bear or Saptarishi (Sapta-seven, rishi-sages). It is a group of seven stars .
Pole Star :-
In ancient times, people used to determine directions during the night with the help of stars. The North star indicates the north direction. It is also called the Pole Star. It always remains in the same position in the sky.
Some celestial bodies do not have their own heat and light. They are lit by the light of the stars. Such bodies are called planets.
The word ‘planet’ comes from the Greek word “Planetai” which means ‘wanderers’.
There are eight planets in our solar system. In order of their distance from the sun, they are:
7) Uranus and
An easy way to memorise the name of the planets in order of their distance from the sun is:
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All the eight planets of the solar system move around the sun in fixed paths. These paths are elongated. They are called orbits.
Mercury is nearest to the sun. It takes only about 88 days to complete one round along its orbit. Venus is considered as ‘Earth’s-twin’ because its size and shape are very much similar to that of the earth. Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (Ceres, 2003 UB313) discovered in recent past may be called ‘dwarf planets.”
Apart from the stars, planets and satellites, there are numerous tiny bodies which also move around the sun. These bodies are called asteroids. They are found between the orbits of Mars and Jupiter .
Asteroids are minor planets, primarily located in the inner Solar System.
The larger asteroids are also known as planetoids. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. The largest of the asteroids, Ceres, was the first to be discovered. A second minor planet – Pallas – was found by Wilhelm Olbers in 1802. This was followed by Juno (1804) and Vesta (1807)
The small pieces of rocks which move around the sun are called meteoroids. Sometimes these meteoroids come near the earth and tend to drop upon it. During this process due to friction with the air they get heated up and burn. It causes a flash of light. Sometimes, a meteor without being comp.
Sun is the closest star to earth-our home planet and is situated exactly at the centre of our solar system. Sun is located at a distance of 149.6 million Km from earth with an average diameter of 696000 Kms. The main constituents of the Sun’s surface are Hydrogen and Helium and continuous fusion of hydrogen to form Helium molecules provides the energy emitted by the Sun.
The visible surface of the Sun is known as photosphere. During a total solar eclipse, when the disk of the Sun is covered by that of the Moon, the Sun's surrounding atmosphere can be seen. It is composed of three distinct parts: the chromosphere, the transition region, and the corona, that together form the heliosphere. The coolest layer of the Sun has a temperature of about 4100 K which exists at about 500 km above the photosphere. The Sun today is roughly halfway through the most stable part of its life. It has not changed dramatically for four billion years, and will remain fairly stable for four billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo severe changes, both internally and externally.
Layers of the Sun (CCPC) :-
Corona is the outermost layer of the sun’s atmosphere.
1)It is full of gas particles although the density of gas particles is very low.
2)It has a temperature of about 17,00,000°C.
One light year is the distance travelled by light in one year at a speed of about 300000 kilometres per second.
The distance of stars from the earth are expressed in terms of light years.
1 light year = 9.46 × 1012 km
The approximate distance of the sun from the earth is 150,000,000 km which means that light takes around 8 minutes and 20 seconds to reach from the sun to the earth.